A see-through, mixed reality display device system enables a user to observe digital information overlaid on the physical scenery. To enable hands-free user interaction, a see-through, mixed reality display device system may further be equipped with an eye tracker. Typically, an eye tracker includes an infrared (IR) light source to illuminate the user's eye and a camera to image the user's eye, e.g., to observe the reflected glints and iris movements for calculation of a gaze direction. The illumination and the imaging of the eye are preferably implemented such that: the see-through properties of the mixed reality display device system are not impaired by the eye tracking hardware; imaging of the eye works with all types of prescription spectacles; and imaging of the eye covers the entire eye movement range plus an inter-pupillary distance range.
One way to image an eye for eye tracking is using a simple camera mounted on the frame of a head mounted display (HMD) device, wherein the camera is directly focused on the user's eye. In other words, there is a direct line of sight from the camera to the eye. While such a configuration is relatively simple and inexpensive, it is highly sensitive to the position and movement of the camera relative to the eye. Also, with such a configuration the camera needs to be positioned close to the eye level, which typically causes at least partial obstruction of the see-through properties of the mixed reality display device system. Alternatively, a partial reflector may be used to fold the camera view path to the user's temple. While this alternative configuration allows the camera to be positioned outside the see-through field, implementation of this alternative configuration is problematic if the eye tracking needs to work with prescription eyewear.
Another possibility is to use a reverse optical path imaging in a free form prism based mixed reality display device system. This technique relies on the actual display optics to also provide the imaging functionality for eye tracking. However, because components of a free form prism tend to be rather large in size, this approach is not always practical. Adding a free form optical for eye tracking only is also possible, but this would be expensive and would add significant weight and size to the system.